Daytime stomatal regulation in mature temperate treesprioritizes stem rehydration at night

Peters, RL; Steppe, K; Pappas, C; Zweifel, R; Babst, F; Dietrich, L; von Arx, G; Poyatos, R; Fonti, M; Fonti, P; Grossiord, C; Gharun, M; Buchmann, N; Steger, DN; Kahmen, A;

Abstract

Trees remain sufficiently hydrated during drought by closing stomata and reducing canopyconductance (Gc) in response to variations in atmospheric water demand and soil water avail-ability. Thresholds that control the reduction ofGcare proposed to optimize hydraulic safetyagainst carbon assimilation efficiency. However, the link betweenGcand the ability of stemtissues to rehydrate at night remains unclear.We investigated whether species-specificGcresponses aim to prevent branch embolisms, orenable night-time stem rehydration, which is critical for turgor-dependent growth. For this, weused a unique combination of concurrent dendrometer, sap flow and leaf water potential mea-surements and collected branch-vulnerability curves of six common European tree species.Species-specificGcreduction was weakly related to the water potentials at which 50% ofbranch xylem conductivity is lost (P50). Instead, we found a stronger relationship with stemrehydration. Species with a strongerGccontrol were less effective at refilling stem-water sto-rage as the soil dries, which appeared related to their xylem architecture.Our findings highlight the importance of stem rehydration for water-use regulation inmature trees, which likely relates to the maintenance of adequate stem turgor. We thus con-clude that stem rehydration must complement the widely accepted safety–efficiency stomatalcontrol paradigm